1. Field of the Invention
The present disclosure relates to an image processing apparatus, an image processing method, and a storage medium.
2. Description of the Related Art
The electrophotographic process is known as an image recording method used for color image forming apparatuses, such as color printers and color copying machines. With the electrophotographic process, a photosensitive drum is exposed to light of a laser beam or light-emitting element, such as a light-emitting diode (LED) to form a latent image. The latent image is developed by toner, i.e., a charged color material to form a toner image. The toner image is transferred to a recording medium, such as paper, and then fixed. A tandem-type color image forming apparatus is an example of an electrophotographic image forming apparatus in which the same number of developing units and photosensitive drums as the number of toner colors sequentially transfer images of different color planes onto an image conveyance belt and a recording medium.
As a technique for reducing misregistration on each color plane arising in a tandem-type color image forming apparatus, Japanese Patent Application Laid-Open No. 2004-170755 discusses a technique for measuring the magnitude of the inclination and a curve of the laser beam scanning line using an optical sensor, correcting image data to cancel the inclination and a curve, and printing the data. This technique includes processing (shifting processing) for shifting pixel data of image data in the sub-scanning direction on a pixel basis. Processing image data through electrical correction in this way eliminates the need of mechanical adjustment members and adjustment procedures in the assembly process. Accordingly, the color image forming apparatus can be downsized, and cope with misregistration at low costs.
Japanese Patent Application Laid-Open No. 4-336859 discusses a technique for reducing an increase in cost of a large capacity storage memory in accordance with the increasing accuracy, resolution, and image processing speed of image forming apparatuses.
The technique discussed in Japanese Patent Application Laid-Open No. 4-336859, using an image forming apparatus having a low print resolution, projects low-resolution image data to expose a photosensitive member to light of the relevant data so that beam dots overlap on exposed portions for pixels. Accordingly, a latent image can be formed such a manner that overlapping exposed portions between pixels serve as effective pixels that can be developed. This technique is referred to as spot-multiplexing for performing image printing with a pseudo higher resolution than that of the image data.
The shifting processing produces a pixel level difference by one pixel to shift the pixel data in the sub-scanning direction on a pixel basis. Therefore, applying the shifting processing to high-resolution image data provides a smaller pixel level difference, which therefore is less noticeable when developed, than applying the shifting processing to low-resolution image data. Accordingly, a sequence is assumed, in which the shifting processing is applied to high-resolution image data and then conversion processing (pseudo high-resolution conversion processing) is applied to the image data (resulting from the shifting processing) to convert it into low-resolution image data for the purpose of spot-multiplexing. In this case, however, in the image data resulting from the resolution conversion processing, the type and profile of appearing halftone dots may change at a certain pixel position in the main scanning direction at which a pixel level difference is caused by the shifting processing. Accordingly, color unevenness may arise in an image printed on a recording medium.